A substantial body of evidence implicates the endogenous opioid system, and the mu opioid receptor (MOR) in particular, in the reinforcing effects of drugs of abuse, including nicotine. A single nucleotide polymorphism (SNP) in the mu opioid receptor gene (OPRM1 Asp40) is associated with the ability to quit smoking, as well as nicotine reward and withdrawal symptoms. However, the precise mechanism through which this SNP influences nicotine dependence remains unresolved. In this R21 application, we propose a translational cross-species approach to elucidate the functional significance of the OPRM1 Asp40 variant in the neurobiology of nicotine dependence. Toward this end, we have developed a knock-in mouse that possesses the mouse equivalent of the Asp40 in the Oprm1 gene (Asp38), and we provide compelling preliminary evidence for functional significance. Understanding whether this variant alters MOR binding in response to nicotine in mice and in human smokers will improve our understanding of genotype by nicotine interactions, and will provide a critical first step toward elucidating the neurobehavioral mechanisms through which this SNP alters smoking behavior. The proposed mice experiments will: 1) determine if the mouse Asp38 alters basal or nicotine-stimulated changes in MOR binding and signaling throughout the brain using [3H]carfentanil and autoradiography;and 2) evaluate the effect of the Asp38 variant on behavioral responses to nicotine using conditioned place preference and nicotine-primed re-instatement paradigms. The human experiment will use [11C]carfentanil PET imaging to assess the effects of intravenous (IV) nicotine versus saline (within-subject) on MOR binding potential in 24 chronic smokers genotyped prospectively and stratified by OPRM1 genotype. These experiments will establish a translational cross-species model for functional characterization of genetic variants, and will elucidate the neurobiology of nicotine dependence, a significant public health problem. PUBLIC HEALTH RELEVANCE: The proposed experiments in mice and humans will help us understand the mechanisms by which a specific gene variant (OPRM1 A118G) is associated with nicotine dependence.